1. Field of the Invention
The present invention generally relates to preparing a semiconductor substrate for device formation and more particularly to junction formation in a semiconductor device. More specifically, this invention relates to ultra shallow junction formation in a semiconductor device and the semiconductor device obtained with the method thereof.
2. Description of the Related Technology
Scaling down of semiconductor MOS devices has become a major challenge in the semiconductor industry. For sub-100 nm MOS devices, highly doped and ultra-shallow junctions are required for the suppression of short-channel effects and the improvement of parasitic resistance. For these ultra-shallow junctions the junction depth may be much smaller than 100 nm (e.g. 25 nm or less). More specifically box-shaped junction profiles with high active dopant concentration are preferred. However the fabrication of such box-shaped ultra-shallow junction profiles is not straightforward. Conventional techniques for junction formation are ion implantation followed by rapid thermal annealing (RTA) to electrically activate the dopant ions. However, the conventional methods are prone to so-called transient enhanced diffusion (TED). During the annealing step (e.g. RTA) the implanted dopant will show an enhanced diffusion due to the presence of excess Si interstitials. The effect of TED can be seen in a broadening of the dopant profile, which is of course disadvantageous for achieving box-shaped junction profiles.
To form an n-type implantation arsenic (As) is a very good candidate due to its abrupt and shallow junction profile. Phosphorus (P) however suffers a lot from this TED effect and the interstitials injected by high concentration P diffusion. To form a p-type implantation typically boron (B) is used. Also this dopant ion suffers a lot from the TED effect, which is especially pronounced in the low concentration region in the tail of the junction. In case of p-type doping there are no straightforward alternatives, like As versus P for n-type doping, which could be used to reduce this problem.
Surface pre-amorphization before dopant ion implantation is widely used to achieve ultra shallow junction profiles. However the main concern with pre-amorphization is the so-called end-of-range damage (EOR) located near the interface between amorphous and crystalline Si. This FOR results in secondary defects which may increase the junction leakage drastically. One way to eliminate these secondary effects is by introducing an additional implantation of carbon (C) after the dopant implantation (Appl. Phys. Lett. 62(3), 303 (1993)). It has also been shown that the C implantation performed after the dopant implantation can also eliminate the TED effect depending on the dose used for the carbon.
When this method of pre-amorphization, dopant implantation and carbon implantation is used in the integration process for MOS semiconductor devices, mainly the EOR forms a problem at the junctions (Appl. Phys. Lett. 84(12), 2055 (2004)).
In U.S. Pat. No. 6,680,250 a solution is proposed to space apart the FOR region from the junction region. After gate deposition, an amorphous region is formed to a depth which is significantly greater than the desired source/drain junction depth. The source/drain regions are implanted with dopant ions and followed by a laser thermal annealing to activate the dopants. Although with this solution the end-of-range defects are located far away from the junction position in the vertical direction, in the lateral direction end-of-range defects are still present in the channel region close to the lateral junction position.
US 2004/0235280 describes a method for forming a shallow Junction in a semiconductor substrate. The method comprises:                preamorphizing a first region of the semiconductor substrate to a first depth,        implanting recrystallization inhibitors into a second region of the semiconductor substrate to a second depth,        partially recrystallize the amorphized part of the substrate (first anneal),        implanting dopants before or after partial recrystallization of the amorphized part of the substrate, and        further recrystallize the amorphized part of the substrate (second anneal).        
The first anneal creates end-of-range (EOR) dislocations at the interface between the amorphous and the crystalline material of the substrate. The first anneal only recrystallizes the part of the amorphous substrate that does not comprise recrystallization inhibitors. However, during the second annealing step in US 2004/0235280, EOR dislocations will be formed at the boundary between the recrystallized part and the remaining amorphized part of the substrate. Hence, EOR dislocations will still be present near source/drain regions.